A variety of occupational, visual tests have previously been proposed for the assessment of various aspects of visual performance. For example, colour vision screening has previously been used as a means for detecting colour deficiencies, and as a means for assessing the severity of a user's colour vision loss.
Colour vision testing has also been used to determine whether a user's vision meets the colour vision requirements for a given occupation (for example: aviation, fire, transport or police services); to assist in the detection of diseases (such as diabetes or multiple sclerosis, for example) that can affect visual performance; to assist in the diagnosis of specific diseases of the eye (e.g., optic neuritis, age related macular degeneration, photoreceptor dystrophies, etc); to facilitate disease management and treatment monitoring, and to enable the monitoring of eye-related side-effects in drug trials.
One illustrative vision test that has previously been proposed is described in a paper entitled “New test to assess pilot's vision following refractive surgery” by C. M. Chisholm, A. D. Evans, J. A. Harlow, and J. L. Barbur (published in: Aviation, Space and Environmental Medicine 2003 May; 74(5): pages 551-559). The test described in this paper assessed the quality of spatial vision by using a Contrast Acuity Assessment (CAA) test at normal levels of ambient light (i.e., photopic vision). In essence, this test assessed the quality of achromatic vision by measuring the smallest luminance contrast needed to resolve and locate the position of a gap in an annulus.
Another illustrative, classic vision test involves the measurement of high contrast Visual Acuity (VA). In this test, a user is asked to locate the orientation of the gap in a Landolt C optotype, and the user's visual acuity is assessed on the basis of the smallest, high contrast Landolt C for which the user can resolve and locate the orientation of the gap. The test is carried out with both bright and dark targets and the results provide a measure of visual acuity similar to that measured with Snellen letter charts in optometric practices, but with improved accuracy and the use of a single target. The test can also be used to assess the effect of “visual crowding” when the test target is surrounded by other targets.
Yet another illustrative vision test is described in a paper entitled “Insights into the different exploits of colour in the visual cortex” by J. L. Barbur, A. J. Harlow, and G. T. Plant. (published in Proc. R. Soc. Lond. B. Biol. Sci. 258 (1353):327-334, 19944). The test described in this paper used CAD (Colour Assessment & Diagnosis) to measure red-green and yellow-blue chromatic sensitivity. The paper also describes how background modulation techniques can be used to isolate the use of colour signals, a prime requirement in colour vision testing.
A further illustrative test (known generally in the art as an “advanced vision test”) is described in a paper entitled “‘Double-blindsight’ revealed through the processing of color and luminance contrast defined motion signals” by J. L. Barbur (published in: Progress in Brain Research, 2004, Volume 144, pages 243 to 259). This paper described a motion contrast sensitivity (MCS) test that involves the measurement of the smallest luminance contrast that a user needs to see motion and to discriminate correctly the direction of movement.
The foregoing tests are usually undertaken by displaying computer generated images to a subject via a very high quality and high definition monitor, typically a cathode ray tube. The subject attends to the images presented on the display and operates an input device, typically a selection of switches, in response to the stimuli they are observing on the screen. For example, in a test where the user might be required to identify the location of a gap in a Landolt C optotype, the input device may comprise four switches and the user may be instructed to operate the switch that corresponds to the quadrant of the image (top left, bottom left, top right or bottom right) in which the gap in the Landolt C optotype is located. Once the user has responded to the particular image being displayed, a new image is presented for the user to respond to, and this process continues until a range of images have been presented and corresponding user responses have been noted. The computer program then determines the user's visual performance based on their responses to the images displayed.
Whilst these systems have been shown to be effective in vision testing and have enabled subjects' visual performance to be accurately assessed, it is generally the case that the equipment (in particular the display) required to perform these tests is typically very expensive and hence tends only to be accessible at selected hospitals or research centres. As the equipment tends not to be universally provided, users can often live a relatively long way from the nearest hospital that has the facility to undertake these tests. Travelling to these hospitals is not too much of a problem for able-bodied users, but can be problematic for less able users who cannot travel so easily. The use of such tests for mass screening of diseases of the eye is therefore very limited.
It is also the case that in less developed regions of the world the cost of the equipment is such that the majority of hospitals simply do not have the funds available to acquire the equipment they would need to implement these tests. One unfortunate consequence of this is that many users continue to endure conditions that could perhaps be treated if their vision were to be properly investigated.
It would be highly advantageous, therefore, if a less expensive solution could be proposed, which solution would be more affordable and hence more accessible to users as it would be more likely to be implemented on a wider scale.
However, whilst an easier and less expensive implementation of such tests would undoubtedly be an improvement to existing arrangements, there will still be those users for whom travel is impossible and those hospitals that are still unable to afford the equipment.
Such problems could be mitigated if a testing system could be devised that utilised commonly available visual equipment (such as a computer monitor for example) for the display of tests to users, as users would then be able to undertake the tests using their own equipment and in their own homes. However, the problem here is that the tests are carefully designed to have particular visual characteristics, and it would be very difficult to ensure that reproduced tests still have those characteristics when the visual equipment used for the purpose is likely to differ widely from user to user.
The present invention has been conceived with the aim of addressing one or more of the aforementioned problems.